I wore heels all the time up until college. In college I had to get used to being on my flat feet again in shoes. The more you wear heels the more you can do it. I would bet your achilles tendon shortens a little. Your center of balance shifts a little when you wear high heals also, so you need to get used to it. For many years I coud alternate easily between flats and heals without any problem.
I know cannot wear heels for a long time because I have some muscle trouble. It affects every muscle in my body. When in one position too long it begins to cramp. If I exhaust a muscle, which happens very quickly now, it beging to weaken and can tremor. Heals leave your foot in a permanent positions while wearing them.
Some women make the mistake of oversmoothing their feet with pedicures. This means feet will likely become sore and tire more easily.
The heavier you are the more difficult it can be to wear them, because their is more weight on your feet.
People with poor posture have more trouble wearing them. Having a straight back kind of lifts the weight away from your feet. Not sure if this is scientifically true, but it sure feels that way.
Currently heels are 4 and 5 inches tall which is an outrage! It simply is not safe. I can run in 3 inch heels, not in 5 inch. It is not attractive to have such focus on shoes in my opinion. I watched the music awards and Gweneth Paltrow was on such tall stilts for shoes she looked the whole time as if she might fall down.